Shu met Pikachu when he found his Poké Ball abandoned in a forest. The Poké Ball had become defective and was releasing Pikachu's electricity everywhere. Shu was able to fight against the current and release Pikachu, after which they became partners.
Pikachu can be willful but will refuse orders from Shu. Shu can speak to Pikachu with his Income device. Like Ash's Pikachu, Shu's Pikachu travels outside of his Poké Ball. Even though Shu and Pikachu are friends, they argue constantly.